In recent years, digital and video cameras and computer-generated images have found wide acceptance. The demand for digital color printers has increased to provide for an acceptable hard copy output for the images captured or generated by such cameras and computers.
Of the various recording methods, the recording apparatus that employs the thermal transfer method using an ink donor ribbon makes it easier to maintain the apparatus. In addition, full-color images of higher quality are obtainable with such apparatus. Typically, there is a reasonable match between the size of the ink donor color panel or patch on the ribbon and the corresponding size of the image to be recorded on the receiver sheet.
Thermal dye sublimation or diffusion printers use heat to cause colored dyes on the ink donor ribbon medium to transfer to a receiver medium that is in intimate contact with the donor ribbon. Over the past 20 years a new printing technology known as “resistive head thermal printing” has emerged. Thermal printers are used for a variety of printing needs, ranging from inexpensive monotone fax printers, to near photographic quality continuous tone color images. The highest quality output is produced by the dye diffusion thermal printer. The thermal printing operation is driven by a thermal print head that consists of a number of resistive heating elements closely arranged along the axis of the head. Between 200 and 600 heating elements are aligned per inch. During the dye diffusion printing process, the thermal printhead is brought into contact with a dye coated donor ribbon (see FIG. 1). A chemically coated receiver sheet sits beneath the donor ribbon. The donor/receiver surfaces are compressed between the printhead bead and an elastomeric drum creating a very small but highly pressured nip contact region.
The high pressure creates the intimate contact between the layers that is necessary for efficient thermal transfer. During printing, each resistive element on the head is pulsed with current in order to create heat. This heat then drives the diffusion process. By manipulating the thermal resistor pulsing scheme one can control the temperature history, and subsequently the amount of diffusion taking place beneath each resistor. In the color dye diffusion process three printing passes are used to overlay yellow, magenta, and cyan dye. The result is a high quality, continuous tone color image.
Most printers which employ this process have the property that once a point of the thermal donor media has been used it cannot be reused, as insufficient amounts of dye remain at that point for a second use. Thermal dye donor media come in standard configurations such as a roll or ribbon composed of a series of interleaved cyan, magenta, and yellow (CMY) panels or patches herein below referred to as a triad of color patches. Thermal donor media also come in standard sizes. An additional panel or patch may also be provided with the series of color patches so as to provide a transparent ink panel or patch for transferring a transparent overcoat to a multiple color image formed on the receiver sheet. The thermal transfer medium including the three color panels or patches and a transparent overcoat panel or patch are referred to hereinbelow as a quad of color patches.
In the field of printing of images, and with regard to U.S. Pat. Nos. 5,132,701 and 5,140,341, there is disclosed a method and apparatus to produce an image on relatively large receivers using a thermal printer having multiple color dye transfer patch triads. In the aforesaid patents, there is noted the problem in thermal printing of printing on a receiver that is longer than the length of the dye transfer patch that is available. Thus, image size has typically been limited to the size of a dye donor film patch used to produce the image. To overcome this problem, the aforesaid patents teach steps of producing a first sub-image with a segment thereof having blank areas which are distributed in accordance with a pattern that does not produce a substantially linear alignment of the blank areas with one another. The second sub-image is produced with a segment thereof having blank areas which are distributed in accordance with a pattern that is complementary to the pattern of the blank areas of the first sub-image.
A problem associated with the methods disclosed by prior art is that the image processing requirements for the printers disclosed in the prior art may be more difficult to implement with efficient image processing time and thus may also require greater CPU time by the host computer. Particularly when used in a kiosk environment, where the CPU is required to implement a number of tasks beyond interface with the printer, it is desirable to reduce the need for reducing the communication time with the host computer and the printer when implementing image processing. It also would be desirable to reduce the likelihood of print variation when producing multiple prints of the same image.
It is therefore desirable to produce large images that are free of visually discernible distortions and which can be produced with conventional dye-donor triad or quad films that provide superior results obtainable using gray level pixels.
The various objects and advantages described herein will become more apparent to those skilled in the art from description of preferred embodiments of the invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate examples of the invention. Such examples, however, are not exhaustive of the various possible embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.